1. Field of the Invention
The instant disclosure relates to a rotation speed control circuit; in particular, to a rotation speed control circuit with function of auto-calibrating rotation speed error.
2. Description of Related Art
In the modern society of information technology, a computer system has become one of indispensable information tools, and whether for a desktop personal computer (PC), a laptop PC, or server, a clock operation of a central processor is increasing, and which results in an increasing heat dissipation, and a demand for heat dissipation appears to be more and more important. In prior art, heat dissipation is performed through maintaining a stable rotation speed of a fan. However, if system voltage changes or be affected by the wind pressure in a closed environment, the rotation speed of the fan may be changed. Therefore, the prior art usually need a rotation speed control circuit to control the rotation speed of the fan via negative feedback configuration of the rotation speed control circuit, so as to avoid over-fast or over-slow of the rotation speed of the fan.
U.S. Patent No. US 2012/0007526 disclose a rotation speed control circuit for controlling the rotation speed of the fan. The rotation speed control circuit comprises a rotation speed detector, a sample and hold element, an error amplifier and a pulse width modulation signal generator. The rotation speed detector, for generating a rotation speed voltage according to a rotation speed signal corresponding to the rotation speed. The sample and hold element is coupled to the rotation speed detector for sampling and storing the rotation speed voltage. The error amplifier is coupled to the sample and hold element for controlling a voltage of a filter capacitor according to the rotation speed voltage and a reference voltage, to adjust an error voltage. The pulse width modulation signal generator is coupled to the filter capacitor for generating a pulse width modulation signal according to the error voltage and a triangle wave. Accordingly, the rotation speed control circuit changes open time of the pulse width modulation signal according to the rotation speed, so as to maintain the rotation speed at the target rotation speed.
However, the charging capacitor disclosed by U.S. Patent No. US 2012/0007526 may generate phenomenon of the aging due to some factors, and capacitance value of the charging capacitor may deviate preset capacitance value of the charging capacitor in actual application, e.g. capacitance value of the charging capacitor decreases from 220 nF to 110 nF. Therefore, phenomenon of the aging of the charging capacitor will indirectly results in error of the rotation speed for the rotation speed control circuit, so as to affect operation of the rotation speed control circuit or the fan system.